In recent years, flat display apparatus (flat displays) are used in many fields and places, and have increasing importance with the progress of information technology. A typical example of the flat display at present is a liquid crystal display (also referred to as “LCD”), and as flat displays based on a displaying principle different from that of the LCD: an organic EL display, an inorganic EL display, a plasma display panel (also referred to as “PDP”), a light emitting diode display apparatus (also referred to as “LED”), a vacuum fluorescent display apparatus (also referred to as “VFD”), a field emission display (also referred to as “FED”), and so on are also actively developed. These new flat displays, which are referred to as self light emitting type displays, are significantly different from the LCD in the following respects and have excellent characteristics absent from the LCD.
The LCD is referred to as light receiving type display, wherein the liquid crystal itself does not emit any light and acts as a shutter for transmitting and shielding an external light to constitute a display. Accordingly, the LCD requires a light source and generally needs a backlight. On the other hand, the self light emitting type display emits a light by itself and thus does not require any other light source. In the light receiving type display such as the LCD, the backlight is always lighted regardless of the mode of displayed information, thus consuming almost the same power as in the full displaying state. The self light emitting type display, on the other hand, consumes power only in a place where lighting is necessary depending on display information, so there is principally an advantage in less consumption of power compare to the light receiving type display.
The LCD attains the state of darkness by cutting off the light of a backlight source, upon which the complete prevention of light leakage is hardly attained, while the self light emitting type display in a state not emitting a light is in the state of darkness, thus easily attaining an ideal state of darkness. Accordingly, the self light emitting type display is also significantly superior in respect of contrast.
Since the LCD utilizes regulation of polarized light by birefringence of liquid crystal, the state of display changes depending on the viewing direction, that is, the LCD has strong viewing angle dependence. This problem, on the other hand, hardly occurs in the self light emitting type display.
Further, the LCD utilizes a change in orientation derived from the dielectric anisotropy of the liquid crystal that is an organic elastic substance, and thus the time of response to an electric signal is principally 1 millisecond or more. On the other hand, the above-described techniques under development utilize transfer of carries such as electron/hole, electron emission and plasma discharge, and thus the time of response to an electric signal is as short as the nanosecond level that is such a high speed as not to be comparable with that of liquid crystal, and therefore, there is none of the problem of an afterimage of a motion picture attributable to the slow response of the LCD.
Among these, the organic EL is particularly actively studied. The organic EL is also called OEL (organic EL) or OLED (organic light emitting diode).
The OEL element and OLED element have the structure in which a layer containing an organic compound (EL layer) is sandwiched between a pair of an anode and a cathode, which is based on a layered structure of “anode/hole injection layer/light emitting layer/cathode” of Tan et al. (see, for example, patent document 1). While Tan et al. use a low-molecular material, Henry et al. use a high-molecular material (see, for example, patent document 2).
The efficiency is improved by using a hole injection layer or an electron injection layer, and the color of emission light is regulated by doping the light emitting layer with a fluorescent dye or the like. The organic EL attains high brightness at a relatively low voltage of 10 V or less, so its application as a lighting apparatus as a substitute for a fluorescent light which is problematic due to use of mercury is also expected.
FIG. 9 is a schematic view showing a fundamental sectional structure of a conventional organic EL element 51.
The organic EL emits a light by applying an electric field between the electrodes to apply an electric current through the EL layer. Conventionally, fluorescence emission occurring upon return from a singlet excited state to a ground state has been utilized, but recent studies enable to utilize fluorescence emission occurring upon return from a triplet excited state to a ground state effectively to improve efficiency significantly.
Usually, the organic EL element 51 is produced by forming a transparent electrode on a translucent substrate 52 such as glass substrate or plastic substrate and then forming a light emitting layer 54 i.e. an EL layer and a counter electrode in this order. Generally, from the work function of the transparent electrode such as ITO relative to the energy level of the EL layer, the transparent electrode is often used as anode 53 and a metal is often used as the counter electrode to form a cathode 55. In the organic EL element 51 described above, an emitted light 58 can be confirmed at the side of the transparent electrode 53. It is known that in the organic EL element 51, a hole injection layer 56 and an electron injection layer 57 are arranged respectively between the EL layer and the electrode as needs arise, and the electrode thereby attaining excellent effects for higher efficiency and a longer life.
In this specification, the hole injection layer and the hole transportation layer, or the electron injection layer and the electron transportation layer, are intended to be synonymous with each other.
Generally, the method of forming the EL layer employs a vapor deposition process using a mask in case of a low-molecular material is used as a material of the EL layer, while when a high-molecular material is used, its solution is applied in an ink jet method, spin coating method, printing method, transfer method or the like.
In recent years, a coatable low-molecular material is also reported. Among the methods described above, the method of vapor deposition of the low-molecular material with a mask is limited because of difficulty in large-sizing of the vacuum apparatus and the vapor deposition mask, so there is a problem of difficulty in large-sizing and in preparation of a large number of sheets by using large substrates. This is not problematic for trial manufacture in the stage of development, but indicates poor competitiveness in the market in respect of tact and cost in the full-scale manufacturing stage. On the other hand, the high-molecular material or the coatable low-molecular material can be formed into a film by a wet process such as an ink jetting method, printing method, casting method, alternate adsorption method, spin coating method and dip coating method. Accordingly, it is less problematic for coping with the large substrate and the coating process is promising as a method of forming the organic EL element.
Now, the method for manufacturing the organic EL element shown in FIG. 9 is described.
By sputtering or vacuum vapor deposition of a transparent electroconductive film of ITO or IZO onto a transparent substrate, a transparent electrode can be produced separately from production of an organic EL. On the transparent electrode, for example, the high-molecular organic EL material described in the patent document 2, that is, PPV (polyphenylene vinylene), is dissolved in an organic solvent and spin-coated. Finally, a metal of low work function such as Al or Ag is formed into a film by vapor deposition to give a cathode.
In the manufacturing method described above, however, since the cathode is formed by vapor deposition, a large-scale vacuum apparatus is necessary only for this step, and for vacuum drawing, the production tact is delayed. As a result, there is a problem that the characteristics of the organic EL material that can be coated to form a film cannot be fully utilized.
Against the problem that the superiority of the coating process for the organic material cannot be fully utilized in forming an electrode by vapor deposition, there is proposed an organic EL element having a cathode formed by melting a metal as well as a method for manufacturing the same (see, for example, patent document 3).
A substance of low work function has an excellent electron injection effect, and in this respect, an alkali metal and an alkaline earth metal are most suitable. Use of an alloy composed of an alkali metal or an alkaline earth metal and another metal in an electron injection electrode for an organic EL element has been proposed for an organic EL element produced by conventional vapor deposition or the like (see, for example, patent documents 4, 5 and 6).
However, all low-melting point metals (alloy compositions) usable as the cathode described in the patent document 3 are alloys containing Sn, and any alloys described therein have a melting point over 160° C., as shown in Table 1 in the patent document 3. The patent document 3 also describes that in addition to those shown in Table 1, metals such as Ga, K, Cs and Rb can be used, but Ga, K, Cs and Rb are metals having very low melting points of 29° C., 63° C., 28° C. and 38° C., respectively.
The patent document 3 supra also describes a method of coating a molten metal onto an anode substrate produced from an EL layer, but does not show any specific method for coating the metal in a heated and molten state. The patent document 3 also describes a method wherein an electroconductive paste is printed on an EL layer and then cured by heating to 175° C. As the electroconductive paste, a silver paste is used, and the melting point of silver is as high as 960° C. In this case, the paste resin is merely thermally cured, and it is evident that the metal silver is not melted.
In organic functional elements such as an organic EL element, selection of the melting point of the metal in the electrode is practically very important. As shown in the patent document 3 supra, a metal of very high melting point and a metal of very low melting point cause the following problem. That is, when the melting point of the metal serving as an electrode is high, the high-temperature stability of the organic material layer at the time of forming the electrode is problematic, and a heating temperature significantly higher than the glass transition temperature of the organic material layer causes a problem of seriously affecting the organic material layer.
On the other hand, when the melting point of the metal serving as an electrode is low, the storage stability thereof as a functional element becomes problematic. In an environment such as inside of automobile in summer, for example, the temperature inside the automobile is significantly increased, and when the organic EL element is used as a display apparatus where a low-melting metal serves as an electrode, there is a problem that the electrode is melted due to high temperatures to break the apparatus.
In the foregoing, the organic EL element has been described, but an organic functional element composed of an organic functional material as an organic material layer and electrodes has the same problem.
The alkali metal and alkaline earth metal in organic EL elements described in the patent documents 4, 5 and 6 are strongly oxidative, combustible and unstable in the air and are thus difficult to handle, so they can be formed into a film only under vacuum by vapor deposition, or the like.
In the techniques described in the patent documents 4, 5 and 6 supra, for example an alloy region containing an alkali metal or an alkaline earth metal is formed in the vicinity of a light emitting layer by co-vapor deposition with plurality kinds of metals as independent vapor deposition sources, to form an electrode by vacuum vapor deposition. Other techniques use an alloy of an alkali metal or an alkaline earth metal and another metal, but the electrode is formed by vapor deposition or sputtering of the alloy as a target material; and although the alloy is used, the electrode is produced by the vapor deposition process or sputtering method. This is because there is a problem that the electrode cannot be produced without using the vacuum vapor deposition process since the melting point of the metal used is high.    Patent document 1: Japanese Patent No. 1526026    Patent document 2: Japanese Patent No. 3239991    Patent document 3: Japanese Patent Application Laid-Open (JP-A) No. 2002-237382    Patent document 4: JP-A No. 9-320763    Patent document 5: JP-A No. 10-12381    Patent document 6: JP-A No. 11-329746